December 06, 2009

Fiber lasers Are Increasing to 50 Kilowatts and More

Diode pumping of a dual-clad fiber laser can be from the end or the side, but light must enter at an angle close to the axis of the fiber so the pump light (blue) can be guided within the outer core. The laser species is doped into the inner core (red), which confines laser emission

Like other diode-pumped lasers, fiber lasers essentially convert the raw low-quality pump laser output into a much higher-quality laser beam that can be used in applications from medicine and materials-working to weapons. The fiber geometry has two important advantages for high-power operation: efficient conversion of pump power into output in a high-quality beam, and good dissipation of the inevitable waste heat.

The active species in most fiber lasers is ytterbium, chosen because the small quantum defect–the difference between energy of pump and output photons–is only about 6% when pumping a 1035 nm Yb-fiber laser on its 975 nm absorption line. In contrast, the quantum defect of a 1064 nm neodymium laser pumped on its 808 nm absorption line is 20%. The small quantum defect helps allow optical-to-optical pumping efficiency to exceed 60% for ytterbium-fiber lasers. With electrical-to-optical conversion efficiency of 50% for pump diodes, that means wall-plug efficiency can reach 30%.

Producing higher powers comes at a steep cost in beam quality. Yusin said IPG’s 50 kW laser has a beam quality M2 of 33, although with 170 kW of power needed to generate the multimode beam, its wall-plug efficiency is higher than the single-mode laser. The output comes from combining the beams from an array of 1.1 kW modules in a single length of 200 µm core output fiber. The whole system is about the size of three large refrigerators

Raw power is far from the only story. Nilsson is working on wavelength conversion and Raman fiber lasers. Others are working on ultrafast fiber lasers